The present invention relates generally to the measurement of electrode contact quality for a multi-electrode diagnostic system capable of acquiring a patient electrocardiogram (ECG), and more particularly, to a measurement system that dynamically measures common mode rejection performance for the purpose of measuring electrode contact quality and accuracy of the ECG signal acquisition.
It is useful for the clinician who is applying ECG electrodes to a patient to know if the electrodes are properly connected to the patient. Prior art methods for measuring electrode contact status fall into two basic categories. One category is the application of a current to each of the individual electrodes individually. The second category is the application of a voltage to the patient that is common to all sense electrodes connected to the patient.
The input impedance of the typical ECG circuit is designed to be very high and, as a result, the application of a signal at each ECG input requires a very high source impedance current source. This current source can be either an alternating current (AC) signal or a direct current (DC) signal. One of the electrodes that is connected to the patient is a reference electrode. Typically, the reference electrode is the right leg electrode in a standard lead placement. The reference electrode is a low impedance connection from the patient to a measuring device configured to evaluate electrode contact quality based on the resulting voltage at each respective electrode. The return path for each current source is through the reference electrode. The voltage seen at each electrode is a function of the voltage drop at the reference electrode and the individual electrode.
A disadvantage of applying AC or DC current sources to each input is that this adds significant circuitry to each input at a point where high impedance design and board leakage is critical. Another disadvantage is that these techniques can incorrectly indicate that a lead wire is connected when it may actually be unconnected, if the input impedance of the cable or input amplifier is degraded due to a hardware failure or board leakage cause by ambient conditions of high humidity.
Another disadvantage of applying a DC current source to each input electrode is that DC currents can generate a voltage due to a current flowing through the electrolyte to metal interface of the electrode, creating a DC offset potential separate from the DC resistance of the skin contact impedance. This electrode potential can then be a source of noise if modulated by patient movement. To minimize DC offset potential, it is necessary to use very small DC currents on the order of 10 nA. However, such small currents are very difficult to implement reliably and board leakage in humid environments can potentially cancel such small currents and cause an incorrect reading. Another disadvantage of the DC current method is that the circuitry cannot differentiate between the DC offset potential of the gel to metal interface and that of the skin to electrode contact impedance. The DC current method is typically limited to detecting that the lead wire is connected but not a determination of the quality of the connection.
Applying a voltage at the reference electrode and measuring this voltage at the input lead wires is a much simpler method of verifying that the input lead wire is connected to the patient. This method can simply look at each lead wire individually to see if the signal amplitude is of an adequate level or can compare two or more lead wires differentially to see how well the common mode signal cancels. The voltage applied to the reference electrode would have to contain an AC component in order not to be confused by the DC offset potentials of each electrode.
A difficulty associated with the method of applying AC signals either as a current source or as a voltage at the reference electrode is the removal of the signals from the desired ECG signals. Pace pulse detection of internal pacemakers is typically a high bandwidth measurement of the input signal. Care must be taken not to affect this pace pulse detection outside the ECG bandwidth as well as not affecting the ECG signal inside the ECG bandwidth. Therefore, use of an AC signal for lead wire contact quality detection adds significant complexity in the signal processing necessary for accurate removal from the data that is then used for ECG and Pace Pulse signal processing.